Barrier Islands, Spits, and Coastal Deposition

Where coasts are low, sandy, and gently sloping rather than rocky, the dominant force shaping them is not erosion of solid rock but the deposition and redistribution of loose sediment, driven primarily by longshore drift. When waves approach a beach at an angle, determined by the prevailing wind and offshore current direction, their swash, the rush of water up the beach, carries sand up the slope at that same oblique angle, but backwash, the water's return under gravity, runs straight back down the steepest gradient; the net effect, repeated by thousands of waves a day, is a slow zig-zag transport of sand along the shore in the direction the waves are approaching from, known as longshore, or littoral, drift. This single process is responsible for building nearly every major depositional coastal landform, and coastal engineers frequently install groynes, low walls built perpendicular to the shore, specifically to interrupt longshore drift and trap sand that would otherwise be carried away, sometimes starving beaches further along the same coast of sediment they depend on.